Comparisons between the fecal microbiota of humans from industrialized and non-industrialized communities indicate a pronounced impact of westernization on the diversity and composition of the human gut microbiota. However, the exact consequences of westernization on community characteristics are still insufficiently understood, and the ecological processes that drive differences have not been elucidated. Here we have compared the fecal microbiota of adults from two non-industrialized regions in Papua New Guinea (PNG) with that of United States (US) residents. Papua New Guineans harbor communities with greater bacterial diversity but lower inter-individual variation. Although the fecal microbiota in PNG and US was largely dominated by shared bacterial lineages, the relative abundance of 25 families, 45 genera, and 230 species-level OTUs differed, and 47 core OTUs in PNG were undetectable in US residents. To gain insight into the ecological mechanisms that cause the observed differences, we quantified community assembly processes in PNG and US microbiomes using a null modeling approach. This analysis demonstrated a significant higher rate of bacterial dispersal in PNG and divergent selective environments in the US. Interestingly, equivalent findings were obtained for other datasets comparing industrialized and non-industrialized microbiomes. Overall, the findings demonstrate a dominant role for microbial dispersal in shapingmore » the human gut microbiota in non-industrialized societies, and point to differential selection pressures across individuals as a major factor shaping microbiomes associated with modern lifestyle.« less